The present invention relates to inductors, and more particularly, to a slope gap inductor for reducing line harmonic currents.
The introduction of personal computers has created a need for low cost and efficient power systems for home use. These power systems are required to meet specific operating parameters, and in particular, should meet predetermined specifications for line harmonic levels. For personal computers, the requirements for line harmonic levels can be found in the EN61000-3-2 LHC requirements published in the International Electrotechnical Commission (IEC) Publication 1000-3-2 (first edition; 1995).
FIG. 1 shows a graph of the worst case harmonic current that is allowable under the EN61000 specification. The graph shows harmonic current normalized to 1 watt where all harmonic peaks happen to be in phase with each other. Typically, the harmonic current would appear more as a sine wave like waveform when the peaks are not in phase.
A conventional apparatus for limiting harmonic current uses a bridge rectifier followed by a boost circuit. Line harmonic correction of a rectified AC supply can result in a large amount of ripple current in the secondary circuits. In the past, keeping the output of these secondary circuits within acceptable ripple limits has required use of very large amounts of storage capacitance.
Techniques for harmonic correction without excessive ripple also include use of two independent conversion stages, a power factor correcting stage and DC-to-DC conversion. Other line harmonic correction techniques include using an auxiliary winding on the isolation transformer that can either be cyclically disconnected, or used with a clamp switch to charge a hold up or bulk capacitor.
A typical solution for improving the line harmonic current is to add an inductor in the input line, where the added inductor enlarges the conduction angle of the AC line current. FIGS. 2a-b show exemplary circuits having an inductor placed in the AC line input. The amount of the line harmonic current suppression achieved by this technique relies on the inductance value.
It is apparent that a higher inductance is required at a lighter load to achieve the same current waveform as full load, as the required current change rate of di/dt is lower at light loads. The inductance requirement is also a function of input power level. A higher inductance is required for a lower input power.
Normally, laminated iron with a fixed air gap is used for the core of the inductor, and the inductance value is almost constant over the expected input power range, as long as the core is not saturated. For a fixed load application, the inductor design can always be optimized at that loading condition. However, in a wide range load application, the inductance is over-designed at the full load. This either requires a bigger inductor or using smaller wire size with more turns to get a higher inductance values for light loads, with the penalty of increasing copper loss at full loads. Step gap inductors have been used in prior art devices but the result is that the inductors of the device can only be optimized for a limited load range.
Therefore, it would be desirable to have a way to reduce line harmonic currents without the addition of extra components that increase the cost and complexity of the power system.
The present invention includes a slope gap inductor to reduce line harmonic current (LHC). For example, the slope gap inductor can be used to reduce LHC in a power supply with. a capacitive load to the AC input line. The slope gap inductor results in a inductance value that varies as a function of current. This current dependent inductance optimally meets the EN61000 requirements at a wide range of loading conditions, e.g., from 50 watts to full power. The inventive air gap design enables the inductor""s inductance value to be determined according to the inductor current. This allows for a reduced number of turns in the windings of the inductor and a minimum core size, since the required inductance (at the same power level) can be much lower than that of a typical uniform gap inductor, where the inductor can only be optimized for a fixed load application. A reduction in xe2x80x9ccopper lossxe2x80x9d in the inductor windings is also thereby achieved because of the reduced number of winding turns in the inductor according to the present invention. Copper loss is defined as the resistance of the windings times the square of the conduction current.
Utilization of a slope gap inductor provided in accordance with the present invention allows efficient circuit operation with a wider range of loads and provides greater reduction in harmonics and ripple currents than possible with conventional design techniques. Furthermore, the reduction in harmonics and ripple currents can be achieved without increased amounts of storage capacitance.
In a preferred embodiment of the present invention, a slope gap inductor for use in an electronic circuit to reduce line harmonic current is provided. The slope gap inductor comprises a first inductor core portion, and a second inductor core portion positioned relative to said first inductor core portion so as to form an inductor gap, wherein the second inductor core portion includes a sloped gap surface that forms a sloped gap portion of the inductor gap having a varying gap height, and wherein the sloped gap surface has a slope value that is selected so that the slope gap inductor generates an inductance value responsive to a level of current in the inductor.